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General Synthetic Route toward Highly Dispersed Ultrafine Pd-Au Alloy Nanoparticles Enabled by Imidazolium-based Organic Polymers Yaqiong Gong, Hong Zhong, Wenhui Liu, Bingbing Zhang, Shuangqi Hu, and Ruihu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16794 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Applied Materials & Interfaces

General Synthetic Route toward Highly Dispersed Ultrafine Pd-Au Alloy Nanoparticles Enabled by Imidazolium-based Organic Polymers Yaqiong Gong,†,‡ Hong Zhong,†,‡ Wenhui Liu,† Bingbing Zhang,† Shuangqi Hu,*† and Ruihu Wang*‡ †

School of Chemical Engineering and Environment, North University of China, Taiyuan 030051,

China. ‡

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. ABSTRACT: Bimetallic Pd-Au nanoparticles (NPs) usually show superior catalytic performances over their single component counterparts, the general and facile synthesis of subnanometer-scaled Pd-Au NPs still remains a great challenge, especially for electronegative ultrafine bimetallic NPs. Here, we develop an anion-exchange strategy for the synthesis of ultrafine Pd-Au alloy NPs. Simple treatment of main-chain imidazolium-based organic polymer (IOP) with HAuCl4 and Na2PdCl4, and followed by reduction with NaBH4 generated Pd-Au alloy NPs (Pd-Au/IOP). These NPs possess an unprecedented tiny size of 1.50±0.20 nm, and are uniformly dispersed over IOP. The electronic structure of surface Pd and Au atoms is optimized via electron exchange during alloying, a net charge flowing resutling from counter anions is injected into Au and Pd to form a strong ensemble effect, which is responsible for remarkably higher catalytic activity of Pd-Au/IOP in hydrolytic dehydrogenation of ammonia borane than

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those of monometallic counterparts.

KEYWORDS: Pd-Au alloy, ultrafine nanoparticles, ensemble effect, ionic polymer, anion exchange INTRODUCTION Supported metal nanoparticles (NPs) have attracted tremendous interests in heterogenous catalysis due to the unique physiochemical properties compared with their bulk counterparts.1-6 Considerable endeavors have been devoted to improve the utilization efficency and catalytic perfromance of metal NPs. One major solution is the syntheis of small size of metal NPs with as possisble as exposure of their constitutional atoms to surface.2, 7-10 Considerable progress has been achieved for suppressing their natural aggregation to larger one through enhancing the confinement effect of the support and the interactions between the support and metal NPs.11-15, It has been found recently that the supported bimetallic alloy NPs show higher catalytic activity than their single component counterparts in a variety of catalytic reactions.16-22 The preferable catalytic performances mainly result from the synergistic interactions between two different metals.17,18 However, their complicated fabrication processes with harsh reaction conditions and/or the use of expensive additives have contradicted the conception of green chemistry. Moreover, most of bimetallic NPs show large particle sizes, and their preparation methods and particle sizes are difficult to control in a subnanometer scale.20, 23-25 There is no report for alloy NPs with average particle size less than 2.0 nm. Therefore, it still remains a great challenge to develop a mild and rapid strategy for facile synthesis of supported ultrafine bimetallic NPs in highly efficient heterogeneous catalytic systems. The supports of metal NPs usually play a crucial role in the formation of small-size metal NPs


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and improvement of their dispersion throughout the catalyst supports.26-29 As a type of emerging organic network materials, the main-chain imidazolium-based organic polymers (IOPs) have held great promises for the stabilization of metal NPs.30-32 IOPs can be prepared readily through a facile quaternization reaction between bis- or multi-imidazolyl derivatives and halomethyl benzenes.30 High dispersion in water render them one of promising supports of metal NPs for water phase reactions compared with traditional aromatic catalyst supports.33 The positivecharged

imidazolium

moieties

and

negative-charged

halide

anions

are

distributed

homogeneously in the polymer networks, both of them have been documented to effectively stabilize metal NPs through coordination interaction and electrostatic effect, respectively, which are considered as an efficient way to inhibit the agglomeration of metal NPs during catalytic reactions.30,33,34 Moreover, the relatively weak interaction between host networks and counter halide anions provides the possibility to exchange with metal-containing precursor anions,35,36 thus resulting in controllable loading and uniform dispersion of metal precursors within IOPs. Despite these promising merits, the use of main-chain IOPs as heterogeneous supports of bimetallic alloy NPs has not been reported hitherto. With ever-increasing concerns for air pollution and energy crises, the search for effective hydrogen storage materials is exigent for the upcoming hydrogen economy.37,

38

Ammonia

borane (NH3BH3, AB) has emerged as one of leading candidates for chemical hydrogen storage because of its high gravimetric hydrogen density (19.6 wt%), low molecular weight (30.87 g mol-1) and high stability under ambient conditions.39-42 To meet its practical application, one of the most challenges is to develop highly efficient heterogeneous catalytic systems based on metal NPs to boost the kinetic and thermodynamic properties under mild conditions.39, 41 Herein, we presented a simple and general method for the synthesis of ultrafine Pd-Au alloy NPs



immobilized by IOPs. The alloy NPs in Pd-Au/IOP possess an unprecedented ultrafine size of 1.50±0.20 nm, and exhibit remarkable catalytic activity, high stability and excellent reusability in hydrolytic dehydrogenation of aqueous AB solution at 25 oC. The excellent catalytic activity mainly results from strong ensemble effect between Au and Pd, while high stability and recyclability rely on the cooperative interactions of homogenous distribution of imidaziloum and counter anions with ultrafine Pd-Au NPs. EXPERIMENTAL SECTION Synthesis of IOP: A mixture of 2,4,6-tris(4-(bromomethyl)phenyl)-1,3,5-triazine (1.0 mmol, 588 mg ) and 1,3,5-tri(1H-imidazol-1-yl)benzene (1.0 mmol, 276 mg) in MeCN (150 mL) was stirred at 100 oC for 24 h. The resultant precipitate was collected by centrifugation, washed with MeCN (3 x 30 mL) and dried under vacuum. Yield: 786 mg (91%). Elemental analysis calculated (%) for C39H33N9Br3: C, 54.36; H, 3.83; N, 14.63. Found: C, 48.24; H, 4.16; N, 13.20. FTIR (KBr cm-1): 3404 (m), 3068 (w), 1617 (m), 1580 (w), 1513 (s), 1416 (w), 1361 (m), 1220 (w), 1182(w), 1013 (w). Synthesis of Pd-Au/IOP: IOP-1 (400 mg) was added into a mixed aqueous solution (20 mL) of HAuCl4 and Na2PdCl4 (1:1) [HAuCl4: 16.99 mg (0.05 mmol), Na2PdCl4: 14.71 mg (0.05 mmol)], the mixture was stirred at 25 oC for 9 h. The resultant product was washed thoroughly with copious H2O to remove excess HAuCl4 and Na2PdCl4, then dried under vacuum at 80 oC for 12 h. An aqueous solution (50 mL) of NaBH4 (37.83 mg, 1 mmol) was added to the suspension of the above polymer in water (50 mL), the mixture was stirred at 25 oC for 2 h. The resultant powders were collected by filtration, washed with copious water for four times, and dried under vacuum at 60 oC for 12 h. Yield: 365 mg (87 %). FTIR (KBr cm-1): 1666 (w), 1617 (m), 1513


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(s), 1416 (w), 1361 (m), 1182 (w), 1013 (w), 807 (w). Synthesis of Pd/IOP: Pd/IOP was prepared using similar method to Au/IOP except that the mixed aqueous solution of HAuCl4 and Na2PdCl4 (1:1) was replaced by Na2PdCl4. Yield: 368 mg (89 %). FTIR (KBr cm-1): 1666 (w), 1617 (m), 1513 (s), 1417 (w), 1361 (m), 1013 (w), 807 (w). Synthesis of Au/IOP: Au/IOP was prepared using similar method to Pd-Au/IOP except that the mixed aqueous solution of HAuCl4 and Na2PdCl4 (1:1) was replaced by HAuCl4. Yield: 371mg (87 %). FTIR (KBr cm-1): 1666 (w), 1617 (m), 1513 (s), 1416 (w), 1361 (m), 1013 (w), 807 (w). General Procedures for H2 Generation from AB: The jacketed reaction flask (25 mL) containing the catalyst (Pd-Au/IOP, Pd/IOP or Au/IOP) was thermostated to the appropriate reaction temperature under strong agitation. A burette filled with water was connected to the reaction flask to measure the volume of the hydrogen gas evolved from the reaction. When 2.0 mmol aqueous AB solution was transferred into the reaction flask under 800 rpm stirring rate for the appropriate time, recording the displacement of water level every minute to measure the volume of generated hydrogen gas. The reaction was stopped when no hydrogen generation was observed. General Procedures for Recyclability Test: After hydrogen generation from aqueous AB solution, the residual catalyst in the aqueous phase was separated by filtration, and washed successively with water and ethyl alcohol. The recovered catalyst was directly used for the next run with the addition of fresh 4 mL aqueous AB solution (0.5 M).



RESULTS AND DISCUSSION

Scheme1. Schematic illustration for the synthesis of IOP and Pd-Au/IOP As shown in Scheme 1, the quaternization reaction between 2,4,6-tris(4-(bromomethyl)phenyl)1,3,5-triazine (TBPT) and 1,3,5-tri(1H-imidazol-1-yl)benzene (TImB) in acetonitrile gave rise to IOP. Subsequent treatment of IOP with mixed aqueous solution of HAuCl4 and Na2PdCl4, and followed by reduction with NaBH4, gave rise to dark grey Pd-Au/IOP. For comparison, IOPsupported Pd and Au NPs (denoted as Pd/IOP and Au/IOP, respectively) were also prepared using aqueous NaPdCl4 or HAuCl4 solution, respectively, under the same conditions. Inductively coupled plasma (ICP) analyses show that Pd and Au contents in Pd-Au/IOP are 0.16 and 0.10 mmol·g-1, respectively, while Pd and Au contents in Pd/IOP and Au/IOP are 0.38 and 0.29 mmol·g-1, respectively.


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As expected, IOP, Pd-Au/IOP, Pd/IOP and Au/IOP show excellent dispersibility in water (Figure S1), which is attributed to the presence of abundant hydrophilic imidazolium moieties and counter halide anions, which is conducive to accelerate interfacial contact between reaction substrates and catalytically active sites.43 The hydrophilicity of IOP, Pd-Au/IOP, Pd/IOP and Au/IOP was further characterized by water contact angle (CA) measurement using pellets of each material (Figure 1).44 IOP shows an excellent water wettability with CA of 53.5o, and the water wettability of Pd-Au/IOP, Pd/IOP and Au/IOP is very close to that of IOP, suggesting that metal NPs loading has no obvious effect on the hydrophilicty of IOP. These results demonstrate that IOP is a promising support of metal NPs for catalytic reactions in water.

Figure 1. Water contact angle for (a) IOP, (b) Pd-Au/IOP, (c) Pd/IOP and (d) Au/IOP. Chemical structures and compositions of IOP, Pd-Au/IOP, Pd/IOP and Au/IOP were identified by Fourier-transform infrared spectroscopy (FTIR), solid-state 13C NMR and elemental analysis. As shown in Figure 2a, the moderate peak at 601 cm-1 is ascribed to C-Br stretching vibration of TBPT, which disappears in the FTIR spectrum of IOP, suggesting complete quaternization of TBPT and TImB.45 In comparison with FTIR spectrum of IOP, a new peak at 1678 cm-1 appears in FTIR spectra of Pd-Au/IOP, Pd/IOP and Au/IOP, which may be attributed to the interaction between imidazolium moieties and metal NPs,46 other peaks are identical with that of IOP, which



indicates structural preservation of IOP after loading of metal NPs. In solid-state

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13

C NMR

spectra of POP-1 (Figure 2b), the resonance peaks at 169 and 52 ppm correspond to carbon atoms of triazine and methylene, respectively. The peaks at 142~118 ppm are assigned to their aromatic carbon atoms. Notably, the solid-state

13

C NMR spectra of Pd-Au/IOP, Pd/IOP and

Au/IOP are almost identical with that of IOP, which further indicates that the structural network of IOP is well maintained after loading of metal NPs.

Figure 2. FTIR spectra for (a) TBPT, TImB, IOP, Pd-Au/IOP, Pd/IOP and Au/IOP; (b) solidstate

13

C NMR spectra of IOP, Pd-Au/IOP, Pd/IOP and Au/IOP; (c) Pd 3d and (d) Au 4f XPS

spectra for Pd/IOP, Au/IOP, Pd-Au/IOP and Pd-Au/IOP-7run. Elemental analyses of IOP show that the experimental values of C and N are slightly lower than corresponding theoretical values, probably due to the presence of trapped guest water molecules, which is common for the imidazolium-based ionic materials.47,48 Notably, the N/C


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molar ratio of 0.27 is the same as the theoretical value. The existence of water molecules is further supported by thermogravimetric analysis (TGA). The initial weight losses of 5.0, 5.5, 6.0 and 7.5 % before 150 oC are observed in TGA curves of IOP, Pd-Au/IOP, Pd/IOP and Au/IOP, respectively. The polymeric networks are stable up to 280 oC (Figure S2).

Figure 3. SEM images for (a) IOP, (b) Pd-Au/IOP, (c) Pd/IOP and (d) Au/IOP. The porous properties of IOP, Pd-Au/IOP, Pd/IOP and Au/IOP were investigated by physisorption of nitrogen at 77 K. As shown in Figure S3, their nitrogen adsorption-desorption isotherm exhibits a type IV pattern according to the IUPAC classification.49 The apparent hysteresis at high relative pressure indicates the presence of extensive mesopores in IOP.26 Brunauer-Emmett-Teller (BET) surface area of IOP is 55 m2 g-1, the low surface area is probably ascribed to framework flexibility of IOP and pores filling by bromine anions. The pore size distribution reveals that IOP possesses predominantly mesopores (Figure S4), which is in accordance with the result of nitrogen adsorption-desorption isotherm. The shape of nitrogen adsorption-desorption isotherms of Pd-Au/IOP, Pd/IOP and Au/IOP is similar to that of IOP



(Figure S3), indicating the pore systems have not been altered substantially after loading of metals NPs.26,27 BET surface areas of Pd-Au/IOP, Pd/IOP and Au/IOP are decreased to 23, 20 and 21 m2 g-1, respectively, which results from partial pore filling and mass increment after loading of metal NPs.27

Figure 4. TEM, HRTEM and corresponding FFT patterns for (a-d) Pd/IOP, (e-h) Au/IOP and (il) Pd-Au/IOP. Field-emission scanning electron microscopy (SEM) images show IOP possesses a wireshaped morphology (Figure 3), which is much different from the granular morphology in reported IOPs.48,50,51 The morphology is well retained after loading of metal NPs. Transmission


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electron microscope (TEM) images of Pd/IOP, Au/IOP and Pd-Au/IOP exhibit that metal NPs are well dispersed on the surface of IOP (Figure 4), the average diameters of Pd, Au and Pd-Au NPs in Pd/IOP, Au/IOP and Pd-Au/IOP are 1.55±0.20, 1.80±0.20 and 1.50±0.20 nm, respectively (Figure S5, S6 and S7). It should be mentioned that the distribution and size of these metal NPs are much more uniform and smaller than those supported by carbon52,53, zeolite54 and metal oxides55, especially for Pd-Au alloy NPs, which probably results from homogeneous distribution of imidazolium moieties and counter anions as well as electrostatic and/or coordination interactions in IOPs.33,34 The HRTEM images and corresponding fast Fourier transform (FFT) patterns show that the intervals between two lattice fringes in Pd/IOP and Au/IOP are 0.225 and 0.239 nm, respectively (Figure 4c,d,g,h), which correspond to (111) plane of Pd and Au NPs.53,56 The crystal lattice fringes with an interval of 0.231 nm are observed clearly in Pd-Au/IOP (Figure 4k, l), which lies between the intervals of Pd NPs in Pd/IOP and Au NPs in Au/IOP. The ultrafine size and homogeneous distribution of Pd-Au alloy NPs on the polymer network were further validated by high-annular dark-field scanning TEM (HAADFSTEM) and Energy-dispersive X-ray (EDX) mapping images. As shown in Figure 5a,b the ultrafine Pd-Au alloy NPs are uniformly distributed over IOP, which are consistent with TEM analyses. The corresponding element mapping images show that the distribution between Pd and Au is identical with each other (Figure 5c), further revealing the alloying state of bimetallic PdAu NPs.52 In powder X-ray diffraction (PXRD) patterns of Pd/IOP, Au/IOP and Pd-Au/IOP (Figure S8), no obvious characteristic diffraction peaks are detected, which is probably attributed to ultrafine size and uniform distribution of metal NPs combined with the amorphous nature of IOP.26



Figure 5. HAADF-STEM (a, b) and EDX mapping (c) images for Pd-Au/IOP. The surface nature of metal NPs in Pd/IOP, Au/IOP and Pd-Au/IOP were examined by X-ray photoelectron spectroscopy (XPS). Their Pd 3d XPS spectra provide two sets of doublet peaks (Figure 2c), which correspond to Pd 3d5/2 and Pd 3d3/2 of Pd(II) and Pd(0) species,53,57 respectively. In Pd 3d XPS spectrum of Pd/IOP, the peaks of the binding energies at 335.41 and 340.67 eV are attributed to Pd(0) 3d5/2 and 3d3/2, respectively, the peaks at 337.20 and 342.46 eV are assigned to Pd(II) species. In Au 4f XPS spectrum of Au/IOP, two sets of double peaks are ascribed to Au 4f7/2 and Au 4f5/2 of Au (δ+) and Au (0) species,53,57 respectively (Figure 2d). The binding energies peaks at 82.78 and 86.48 eV are assigned to Au(0) 4f7/2 and 4f5/2, respectively, while the peaks at 84.18 and 87.88 eV correspond to Au (δ+) 4f7/2 and 4f5/2, respectively. Impressively, the Pd3d5/2 binding energy peak of Pd(0) species and Au 4f7/2 binding energy peak of Au(0) species in the XPS spectrum of Pd-Au/IOP shift negatively by 0.83 and 0.30 eV, respectively, when compared with those in Pd/IOP and Au/IOP, respectively. The lower binding


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energies for both Pd 3d3/2 and Au 4f7/2 are attributed to electron exchange between Au and Pd.58,59 Au gains s, p electrons of Pd, and the concerted depletion of Au 5d electrons partially compenates this gain of charge, which results in net charge flowing into Au (higher Au s or p electron densities) and Pd (gaining d electrons from Au).58,59 It should mentioned that the ensemble effect between Pd and Au in Pd-Au/IOP is quite different from the electron-transfer effect from Pd to Au in most reported Pd-Au alloys.52,57 The span of the binding energies shift in the XPS spectra usually reflects the synergism extent of Au-Pd alloy NPs.53 The maximum shifts of -0.30 eV for Au 4f7/2 and -0.83 eV for Pd 3d5/2 peak in Au-Pd/IOP are much higher than reported those supported by zeolite

59,60

and metal oxides.61 The abundant negative-charged

counter anions are probably responsible for strong synergism between Pd and Au, they can supply electrons to Pd-Au NPs, which strengthens the alloy effect of Au-Pd and enhances the catalytic performance. The catalytic activities of Pd/IOP, Au/IOP and Pd-Au/IOP in hydrolytic dehydrogenation of AB were investigated in a water-filled graduated Buret system.62-65 Figure 6a depicts the volume of H2 generation as a function of reaction time using different catalysts. Pd-Au/IOP shows the highest catalytic activity with complete AB hydrolysis within 12 min, and 131 mL H2 is produced. However, Pd/IOP only affords 46 mL H2 at 12 min under the same conditions, a full conversion of AB can be achieved when the reaction time is elongated to 32 min. Unexpected, Au/IOP has negligible catalytic activity in AB hydrolysis. The control experiment was performed in the presence of only IOP, no H2 generation was detected. To further elucidate the synergetic effect of Pd-Au alloy, catalytic test of physically mixed Au/IOP and Pd/IOP was also performed under the same conditions, the catalytic activity is very close to that of monometallic Pd/IOP. The results have demonstrated that IOP-supported Pd-Au alloy NPs could achieve a



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remarkable enhancement for the catalytic activity in hydrolytic dehydrogenation of AB due to the strong synergism between Au and Pd in Au-Pd/IOP. The effect of Pd/Au molar ratios on the catalytic activity was further investigated. As shown in Figure S9, the catalytic activity of hydrolytic dehydrogenation of AB slightly decreased when the molar ratio of Pd/Au was increased from 1.60 to 3.76, but a significant deactivation was observed when the molar ratio of Pd/Au was decreased from 1.60 to 0.61. The plot of H2 generation versus catalyst amount was also studied at 25 oC within 12 min, a linear relation between H2 generation and catalyst amount was shown (Figure S10), which reveals that IOP is a promising support, and transport limitation of substrates can be excluded in this catalytic system.66 The effect of AB concentration on H2 generation rate was also studied with constant AB amount. The plot of H2 generation rate versus AB concentration is shown in Figure 6b. No obvious variation of H2 evolution rate is observed when the concentration of AB is increased from 0.25 to 1.0 M, which indicates that the catalytic reaction is zero-order with respect to AB concentration.39 The plots for hydrolytic dehydrogenation of AB at different temperatures were shown in Figure 6c, the H2 generation rate increases sharply when the temperature is enhanced from 25 oC to 50 oC, and the reaction can be completed within 2 min at 50 oC. The total turnover frequency (TOF) of Pd-Au/IOP is calculated to be 25.0 molH2 molcat -1 min

-1

at 25 oC, which is

much higher than that of monometallic Pd/IOP and Au/IOP under the same conditions, and is comparable with those in reported alloy nanoparticles containing Pd and/or Au (Table S1).67-71 According to the Arrhenius plot,72-75 the activation energy (Ea) of hydrolytic dehydrogenation of AB was calculated to be 52.46 kJ mol-1 (Figure 6d).


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Figure 6. (a) Plots of H2 generation from AB as a function of time; (b) plot of H2 generation rate versus AB concentration catalyzed by Pd-Au/IOP; (c) plots of H2 generation from AB catalyzed by Pd-Au/IOP at different temperatures; (d) Arrhenius plots and TOF values of H2 generation catalyzed by Pd-Au/IOP. (e) The recyclability in hydrolytic dehydrogenation of AB catalyzed Pd-Au/IOP at 25 oC. Reaction conditions: Reaction conditions: 0.5 M aqueous AB solution (4 mL), molar ratio of [Pd-Au]/AB = 0.01. The durability of Pd-Au/IOP in hydrolytic dehydrogenation of AB was evaluated using 0.5 M aqueous AB solution at 25 oC (Figure 6e). After finishing the reaction, the residual solid was separated and reused for the next run with the recharge of aqueous AB solution. Interestingly, complete hydrolysis of AB within 12 min can be maintained at least 7 runs. After first run, the



aqueous phase was analyzed by ICP, either Pd or Au leaching was negligible. The excellent recyclability of Pd-Au/IOP is attributed to synergetical coordination and electrostatic interactions of Pd-Au alloy NPs from imidazolium and counter anions.

Figure 7. TEM (a, b), HRTEM images (inset), AADF-STEM and EDX mapping images (c) of Pd-Au/IOP-7run. In order to further understand this catalytic system, Pd-Au/IOP was isolated after consecutive reaction for seven runs, and the resultant sample was denoted as Pd-Au/IOP-7run. SEM images show that the original wire-shaped morphology of Pd-Au/IOP is intact after consecutive AB hydrolysis reaction (Figure S11). TEM analyses indicate that the average size of Pd-Au alloy NPs in Pd-Au/IOP-7run is slightly increased to 2.2±0.20 nm (Figure 7a and Figure S12). The concomitant formation of a few large NPs with an average diameter of 8.2±0.20 nm occurs owing to Ostwald ripening process during consecutive catalytic reaction,76,77 but no obvious agglomeration is observed. In HRTEM images of Pd-Au/IOP-7run, the interval of 0.233 nm


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between two lattice fringes is very similar to that in Pd-Au/IOP, indicating outstanding durability of Pd-Au alloy structure supported by IOP (Figure 7b inset), which is further confirmed by HAADF-STEM and EDX mapping images of Pd-Au/IOP-7run (Figure 7c). Pd-Au/IOP-7run was analyzed by XPS to elucidate the properties of Pd-Au alloy NPs, the ratios for Pd(0)/Pd(II) and Au(0)/Au(δ+) in Pd-Au/IOP-7run are increased from 1.52 to 4.84 and 0.99 to 1.31, respectively (Figure 2c,d), which may be attributed to further reduction of residual Pd(II) and Au (δ+) by generated hydrogen in the process of AB hydrolytic dehydrogenation.26, 27 A plausible mechanism for hydrolytic dehydrogenation of AB catalyzed by Pd-Au/IOP is proposed.78, 79 As shown in Figure S13, the interactions between electronegative Pd-Au alloy NPs surface and AB molecules produce activated complex species, subsequent attack by H2O molecules leads to the concerted dissociation of the B-N bond. The consecutive hydrolysis of each BH3 intermediate releases three equivalent H2 molecules with the concomitant formation of BO2-. In the catalytic cycle, the activation of AB molecule is rate-determining step, large amount of negative-charged counter anions in IOP may donate electrons to Pd-Au alloy NPs, which is beneficial to strengthen the synergism between Au and Pd, resulting in enhancement of the catalytic activity.

CONCLUSION A main-chain IOP with a wire-shaped morphology has been presented, IOP can serve as a promising heterogeneous support of bimetallic alloy NPs. The ultrafine Pd-Au alloy NPs can be facilely generated through an anion-exchange strategy. The homogeneous arrangement of imidazolium and counter anions in Pd-Au/IOP not only endows outstanding dispersibility of alloy NPs over IOP, but also effectively stabilizes Pd-Au alloy NPs by electrostatic and/or



coordination interactions. These Pd-Au alloy NPs possess an ultrafine average size of 1.50±0.20 nm. To our best knowledge, the size is the smallest in the reported bimetallic alloy NPs. The most striking, Pd-Au alloy NPs show a rare ensemble effect, abundant negative-charged counter anions in IOP significantly strengthen the synergism between Au and Pd, resulting in significant promotion of catalytic performance. In summary, this study provides a general strategy for facile synthesis of highly dispersed ultrafine bimetallic alloy NPs with a strong ensemble effect. The synergetic interactions disclosed in Pd-Au/IOP make an inspiration for the stabilization of alloy NPs and the improvement of catalytic performances. Further study for other bimetallic and even multimetallic alloy NPs with their size controllable on a subnanometer scale is on progress. ASSOCIATED CONTENT Supporting Information. The dispersibility in water, TGA, XRD, SEM, TEM, AADF-STEM and EDX mapping images of IOP, Au/IOP, Pd/IOP and Pd-Au/IOP, the N2 isotherms and pore size distribution of IOP, Au/IOP, Pd/IOP and Pd-Au/IOP at 77 K. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail (S. Hu): [email protected] * E-mail (R. Wang):[email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors acknowledge National Natural Science Foundation of China (21603228, 21501155 and 21471151). REFERENCES 1.

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General Synthetic Route toward Highly Dispersed Ultrafine Pd-Au Alloy Nanoparticles Enabled by Imidazolium-based Organic Polymers.


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